A method of this kind is disclosed, for example, in patent specification EP 0 422 255 B1. This publication discloses an electronic ballast for operating fluorescent lamps which enables the brightness and power of the fluorescent lamps to be regulated by varying the switching frequency of the inverter switches. In order to prevent the fluorescent lamp from extinguishing if brightness is low, i.e. if the lamp is operating at only 1% of the nominal luminous flux, not only the power but also the instantaneous discharge resistance of the fluorescent lamp is monitored and a secondary controlled variable is derived from the discharge resistance, which increases as the brightness of the fluorescent lamp decreases, for controlling the inverter switches.
It has been shown that in the case of fluorescent lamps, fluctuations in the operating state or unstable operating states arise if their luminous flux is regulated by means of the abovedescribed method to approximately 25% to 10% of their nominal luminous flux. The reason for these unstable operating states is a nonlinear dependence of the power consumption of the fluorescent lamp on the frequency of the current generated by the inverter. In an unfavorable case, even the smallest of changes in the switching frequency of the inverter and thus in the frequency of the current flowing through the bridge circuit can cause severe changes in the lamp power in the abovementioned range.